[Baertram]

ZO_QuickSlot does not exist any longer thus the texture MyTexture that wants to be added to it cannot find it and cannot be created -> the texture MyTexture will be nil in the end and thus the error tells you that something will index (find an attribute or variable) of a nil table (MyTexture).
With High isle the Quickslots were changed and now you got QUICKSLOT_KEYBOARD instead, and the control of the quickslots is QUICKSLOT_KEYBOARD.control I think.